1. Field of the Invention
The present invention relates to a method for supporting a catalytic metal on the surface of a carrier, and particularly relates to a method for controlling the supported depth of a catalytic component within a suitable range.
2. Description of the Related Art
A supported catalyst in which a catalytic component is supported on the surface of a carrier having a large specific surface area has been industrially widely used for economical reasons that the amount of the catalytic component to be used can be reduced by supporting the catalytic component with a high degree of dispersion.
In such a supported catalyst, it is the catalytic component on the surface of the carrier that contributes to a catalytic reaction, and the catalytic component in the inner part of the carrier can hardly contribute to a catalytic reaction. Consequently, if the catalytic component in the inner part of the carrier which cannot contribute to a catalytic reaction reaches an amount that cannot be ignored as compared with the amount of the catalytic component on the surface of the carrier, problems are that the loss of the catalytic component will occur and that the reaction efficiency per the amount of the catalytic component will be reduced. Particularly, in the case where an expensive precious metal or the like is used as a catalytic component, the cost for producing a practical supported catalyst will be increased because the catalytic component is also supported in the inner part of the carrier.
Accordingly, in order to increase the reaction efficiency of the supported catalyst, it is desirable to support the catalytic component only on the surface of the carrier and its vicinity, and several methods for producing such a supported catalyst have been proposed.
One of these methods is a method of impregnating a porous carrier with an acetone solution or an acetone-water mixed solution of a palladium compound and carrying palladium metal or a palladium compound on the surface part of the porous carrier, as described in Patent Literature 1. According to this method, 97% of the palladium catalyst can be adhered to a region from the surface of the carrier to a depth of 0.2 mm or less (Patent Literature 1, Example 1).
Further, as described in Patent Literature 2, there is a method of placing a carrier supporting a basic metal salt component of at least one metal of group 1 metals or/and group 2 metals of the periodic table into a solution containing a palladium compound and a tellurium compound held at a temperature of 60° C. or more and 100° C. or less to bring them into contact with each other so as to carry palladium and tellurium on the carrier. According to this method, a phenyl ester-synthesizing catalyst can be produced in which the amount of palladium and tellurium carried in a surface layer part ranging from the outside surface of a carrier to a depth of 20% of a radius to the center of the carrier is 70% or more of the total amount of palladium carried and 50% or more of the total amount of tellurium carried, respectively (Patent Literature 2, Claim 1).
However, it is difficult to completely control the supported state of the catalytic component by the method of Patent Literature 1, and the catalytic component cannot be prevented from diffusing into the inner part of the carrier. Accordingly, even the above method allows about 3% of the palladium catalyst to be diffused into the inner part of the carrier, and it is difficult to control the localized depth of the catalytic component from the surface of the carrier.
Also, with respect to the method described in Patent Literature 2, it is difficult for the catalyst present in a part ranging from the outside surface of a carrier to a depth of 20% of a radius to the center of the carrier to sufficiently contribute to a catalytic reaction. Therefore, the catalyst is required to be supported in closer vicinity of the surface. Further, about 30% of the total amount of palladium carried will diffuse into the inner part of the carrier, and the effect of the catalyst is not sufficient in consideration of production cost and catalytic activity.
The present inventors have disclosed a method described in Patent Literature 3 against the prior art as described above. This method includes preliminarily impregnating a carrier with a specific liquid compound to allow the compound to diffuse into the inner part of the carrier and then solidifying the compound in the inner part of the carrier, before bringing the carrier into contact with a catalytic metal salt (Patent Literature 3: Claim 1). This method can prevent the catalytic component from diffusing into the inner part of the carrier and allows the catalytic component to be supported only on the surface of the carrier and its vicinity.
The above method by the present inventors is superior in that the catalytic component is supported only on the extreme surface of the carrier. However, in this method, it is necessary to solidify the impregnated compound in the inner part of the carrier, and it is also necessary to remove the solidified compound from the inner part of the carrier after the catalytic component on the surface of the carrier is immobilized. For this reason, such a process of producing a supported catalyst includes relatively complicated steps.